1. Field of the Invention
The present invention concerns a synchronization system and a process for audiovisual programmes, as well associated devices and processes. It pertains in particular to units and to processes for recognizing and specifying synchronization signals.
Interactive television allows a viewer to act on the course of the transmissions that he is watching. He can thus interact with interactive services. To do this, interactive mass-market terminals are connected to a dual communication network:                a bidirectional network (modem return path, cable, etc.) of point-to-point type, such as in particular a telephone network, implementing one or more point-to-point servers,        and a unidirectional distribution network, such as in particular a terrestrial, cable or satellite TV transmission network, implementing one or more broadcasting servers; the expression “broadcasting” is understood to mean the transmission of identical data to a set of destinations, whether this transmission be performed in particular by radio transmission, by cable or by Internet.        
The terminals considered consist for example of television receivers, of DVB (standing for “Digital Video Broadcasting”) decoders or Internet decoders. The interactive services are generally downloaded into the terminals by broadcasting via the unidirectional network. In other cases, they are hosted by servers that can be accessed by the telephone network. These services are then accessed through references or web addresses (also called URLs standing for “Universal Resource Locators”, or universal addresses) transmitted as an addition to an audiovisual programme. They determine the server to which the terminals should address themselves as well as the service to be invoked on this server. The audiovisual programmes transmitted synchronously either with the services, or with URLs of the services, are then designated by “enhanced programmes”.
2. Description of the Prior Art
In practice, the known techniques of interactive television rely on the operations which follow. An interactive service emanating from a services operator, is transmitted by means of a broadcasting centre, or “broadcaster”, in a manner synchronized with an audiovisual programme (unidirectional network). For this purpose, data relating to this service are embedded with audiovisual signals specific to this programme. When an interactive terminal receives the enhanced programme thus obtained, it transmits to the screen (picture and sound) the audiovisual programme and interprets the data of the interactive service. It then screens the latter in synchronization with the audiovisual programme, typically in the form of a graphical or textural display overlaid on the video.
A viewer can then interact with the interactive service, this interaction possibly leading to the setting up of a connection with a server of the services operator (bidirectional network).
One of the essential aspects of interactive television relates to the establishment and the management of synchronization between the programmes and the interactive services data. In particular, provision is generally made to transmit the interactive content or its URL in a loop (carousel) for the duration of the associated audiovisual programme. Two items of equipment of the broadcaster are customarily used for the synchronization:                a traffic system, which creates an events execution list (“playlist”) comprising time information (start time and end time), object information (device to be driven) and operation information (command to be executed);        and an automation system, which manages the execution list so as to drive and command the broadcaster's transmission equipment (video servers, video recorders, subtitling apparatus, etc.).        
The traffic system makes it possible to make modifications to the execution list in case of significant modifications in the temporal organisation of the programmes. The automation system is for its part capable of rejigging the list in case of last-minute tiny modifications, and of redirecting the list to the traffic system if more consequential modifications are required.
The synchronization of the interactive services with the programmes is obtained by integrating a broadcasting server (which acts as server of interactive applications) among the devices to be driven by the automation system. The service operator is assumed to be connected permanently to the broadcaster's transmission server, so as to be notified of the effective transmission of the programmed contents. This involves a link of the TCP-IP type (standing for <<Transmission Control Protocol/Internetwork Protocol>>) or the like on which a layer of a particular application package may be deployed. This link serves in one direction to programme the transmission server on the basis of administration applications present at the service operator, and in the other direction to inform the services operator in particular of the state of the transmission server, of the progress of transmissions of contents and of any incidents.
Problems of desynchronization between the transmission of the programmes and those of the associated interactive contents are thus avoided. Specifically, if an audiovisual programme is delayed or advanced, the execution list is updated by the broadcaster. In this way, the devices used—including the transmission server—trigger at the appropriate moments.
However, such a technique requires the intervention of the broadcaster, who has to modify his events execution lists to take account of events related to the transmission of interactive services. Moreover, it requires an agreement between the broadcaster and the services operator, so that they concur with regard to commands relating to the transmission of such services.
Various methods have also been proposed in which synchronization signals are inserted into the audiovisual programme itself. Thus, document WO-01/50764 describes a computer process for the utilisation of an interactive digital television transmission, in which service signals corresponding to “synchronization pulse” sequences are detected, which give rise to the downloading of a multimedia application. In U.S. Pat. No. 5,818,440, an interactive application is downloaded into an interactive television network, and this application is automatically enabled upon the detection of an application token incorporated into the video programme.
Such embodiments likewise require the intervention of a transmitter or of a service provider for inserting the stream of appropriate synchronization signals, and therefore have an intrusive nature.
These difficulties are resolved by the intervention disclosed in document WO-01/91462. This priority document describes a device for synchronizing audiovisual programmes transmitted and complementary information. The device comprises an assembly for detecting pictures and/or sounds, which is capable of extracting at least one semantic element from the content of an audiovisual programme currently being transmitted. It also comprises a unit for recognizing these semantic elements, which is linked to a programme guide which comprises a chronologically ordered list of information sets associated with the audiovisual programmes. The recognition unit, prepared by prior learning, selects the information set which is most probably correlated with these semantic elements. A synthesis block then synchronizes the audiovisual programmes with the information sets selected.
However, this invention necessitates complex means and requires reference to a programme guide.
Patent application WO-02/21840 proposes a method of synchronizing a client with a media signal. According to one embodiment of this method, one or more actions corresponding to an audiovisual signal (the media signal) is or are received, an application for performing the actions (for example by syntactic analysis—or “parsing” —of action information) is determined, and an enabling of the actions is triggered by this application. In the examples described, an operator defines actions that he despatches to a server, which itself transmits them by broadcasting via Internet to clients. Either the actions despatched have to be executed upon receipt, or they are accompanied by temporal information which makes it possible to determine the moments at which to execute them.
A drawback of this method is that it necessitates relatively complex operations for the synchronization, either in the form of interventions of an operator at the requisite moments, or in the form of a prior preparation making it possible to trigger the despatches of the actions at the opportune moments.
Document WO-01/82623 discloses the automatic insertion of interactive TV triggering messages (“triggers”) into a stream of audiovisual data. The insertion can in particular be based on the recognition of particular elements in the stream, such as audio/video samples stored in a database. Moreover, this insertion may be triggered before or after broadcasting the stream. The description also mentions the use of a tagging table comprising relations between on the one hand the samples to be found, and on the other hand interactive elements and associated attributes, such as for example the station number.
However, this very flexible technology presupposes prior acquisition of the recognition elements, which have to be made available for the automatic insertion system. But, the priority document WO-01/82623 does not describe the mode of obtaining these elements. A person skilled in the art seeking to implement such acquisition could then envisage several methods.
A first method of acquisition would apply in the case of an insertion of the triggers upstream of the broadcasting. An operator would view the programmes concerned in advance, would select significant elements to be recognized and would record them in a base accessible directly by the automatic insertion system. This type of embodiment is however restricted to insertions of triggers before broadcasting, and involves tight communication between the operator and the broadcaster.
A second method of acquisition would consist in recording the recognition elements in a portable storage medium, such as for example a cassette, a hard disc or a DVD (standing for “Digital Versatile Disc”), then in making the medium available to the user of the automatic insertion system. This method has with respect to the first the advantage of being applicable in a broader context, and can theoretically be utilized not only by a broadcaster, not necessarily benefiting from immediate access to storage resources common with the operator, but also by an end user of a video decoder. However, it constrains storage media to material transmissions and does not permit flexible modifications of the recognition elements. Its practical field of application therefore also turns out to be fairly restricted.
A third method of acquisition would consist in transmitting the recognition elements by teletransmission, via a communications network. This solution, which offers great flexibility and makes it possible to utilize a system of automatic insertion both upstream and downstream of the broadcasting, is however penalised by the transmission resource requirements. Indeed, to be significant, the recognition elements are in numerous cases at risk of being relatively voluminous, pertaining to portions of audiovisual contents. Their transmission then entails significant costs in terms of bandwidth, encroaching on the communication resources. For example, to transmit complete pictures in an analogue television environment, a bandwidth of only a few tens of k-bytes/second is available.